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The fields NUM & AMT have 2 dec. Unless rate is defined with more decimals (in IMG), it also has 2 dec.
Each time you multiply or divide will result in rounding off. There is no intermediate results kept with higher accuracy in these fields, unless you multiply initially by 1000 (as done usually in PCR's), then round off at the end of all calculations. The rounding can be normal r/o off to nearest, round down or up. These r/o rules can be specified in the rounding op'n (e.g., ROUND), or a custom op'n.

Often r/o diff occurs in a retro run, when, for example, an absence record is deleted or changed.

The reason for this happening is because your calculation is happening in two steps:-
1. The overtime rate of 1.5 is multiplied to 11.11 which equals 16.665 which is rounded up to 16.67
2. The number of overtime hours is then multiplied to this which equals 16.67 * 4 =66.68 and that's how you get the value.

You need to see the documentation for the ROUND operation (transaction code pe04)

Let us assume that your Time Wage Type X has the Rate (R) and Amount (A) and you use MULTI ARA where you multiply the Rate and Amt and store the multiplied value in A ( Amount)
Next you need to use the ROUND function to ROUND up or down and there are different options :-